1. Field of the Invention
This invention relates to optical fiber and optical sensors and, more particularly, to a unique combination of wavelength modulated optical sensors coupled to a spectrograph detector and an optical fiber transmission link.
2. The Prior Art (The citations are expanded below)
Conventional oceanographic instruments make use of electrical sensors and electrical wires and use a time consuming measurement process. With research vessel time becoming more expensive and difficult to get, the necessity for accurate expendable oceanographic sensors and probes is becoming more apparent. Present expendable technology makes use of electromagnetically active wires and sensors and are produced primarily by Sippican Corporation of Marion, Mass., Magnavox Inc., and Sparton Inc.; the latter two are involved in the manufacture of expendable acoustic hydrophones called sonobuoys. Sippican Corporation has manufactured the expendable bathythermograph since 1965; this is called an XBT and measures the temperature and time as the device falls through the ocean. Time is theoretically related to depth and the information from the thermistor is relayed back to the surface vessel through a thin wire that unreels from the probe vehicle as it falls. This wire breaks when it is unreeled and the measurement is complete; the probe vehicle and sensor then descend to the bottom of the ocean. Sippican also manufactures expendable sound velocimeters (XSV, 1979) that measure the speed of sound and time, expendable current profilers (XCP, 1983) that measure the magnetic field and time, and expendable air launched bathythermographs (AXBT, 1984) that are dropped from aircraft into the ocean; an expendable conductivity, temperature, and depth probe (XCTD, 1985) is now being developed to measure conductivity, temperature, and time. The expendable current profiler was originally developed in a non-expendable form by Sandford and Drever in 1978 at the Woods Hole Oceanographic Institution; also other versions of the expendable air launched bathythermographs are manufactured by Magnavox Inc., and Hermes Inc.
Electrical expendable probes, transmission links, and sensors suffer from the following problems:
1. Depth sensing is intractable,
2. Conductivity, and therefore, salinity measurement is difficult and unreliable,
3. Failure rate is high due to electrical insulation leaks,
4. The thermal sensor time constant is large, and
5. Wire transmission link data rate is low.
The simple design and rugged and electromagnetically passive nature of optical fibers and optical sensors offers solutions in these areas. The present electrical expendable instruments measure time and, assuming a constant free-fall velocity, relate it to depth; this method has an average error of 31/2% with an even greater error for the deeper probes. Optical pressure sensors used to measure depth have accuracies generally at 0.4%. Optical index of refraction sensors do not have the drift and instabilities created by the films and polarization encountered in electrical conductivity probes. Also, this electrically passive nature eliminates the failures caused by electrical insulation breaks and the radio frequency pick-up in the probe and transmission link of conventional expendables. The ocean is also an electrical conductor. In the past data from expendable instruments has not been fully trusted by oceanographers except, perhaps, for survey work. Noncatastrophic wire insulation leaks result in signal errors that are not immediately apparent and require elaborate screening procedures for the data to be believed. In regards to the thermal time constant, the absence of electrical insulation covering the thermal sensor offers an improved time response potential over the electrical thermistor. Finally, the optical fiber transmission link is capable of passing 200 megabits/sec of data, enough for a hundred or more sensors, as opposed to the two or three limit imposed by thin electrical wire.
In the past the cost of optical fiber has been prohibitive for its use in expendable probes. Seven years ago its cost was $1.50 per meter; today the retail quote is $0.15/m with wholesale discounts beyond this, and the prospect is for the price to continue to decrease. The manufacturers, such as Corning Glass Works, have stated that the long term goal is to make glass fiber equivalent to copper wire in price.
In addition to the above oceanographic and underwater sensing applications, remote optical sensing has application in industrial process control, cryogenic environments, and in fiber optic data- and tele-communications. Evanescent wave spectroscopy and liquid chromotography are two industrial applications of fiber optic refraction sensors as described by Lew, et. al. (1984) and David et. al. (1976) respectively. The use of optical pressure, temperature, and refraction sensors to avoid electrical hazard in explosive environments is discussed by Sharma and Brooks (1980). Finally, local area data and tele-communication networks are increasingly using optical fibers, and optical pressure, temperature, and liquid level refraction sensors are a needed addition for such purposes as building security as discussed by Harmer (1983).
In surveying the specific optical sensing techniques presently in use, we find that those sensors that use amplitude modulation are not sensitive enough and have drift and calibration problems, whereas other optical sensors that use phase modulation are sensitive to too many factors, particularly in remote applications. Christensen (1979) has developed a band edge semiconductor temperature sensor that is amplitude modulated; the drift is only partly compensated for by using a reference signal, and the instrument must be recalibrated every few hours. Also, Spillman and McMahon (1982) have developed a birefringent pressure sensor which is also amplitude modulated, and Mahrt, et. al. (1982) has developed an in-situ critical angle refractometer that has a wire link return and is not expendable. In regards to phase modulation the Naval Research Laboratory in Washington, D.C. has developed interferometers for optical acoustic pressure sensing in the oceans; they have been able to attain very high sensitivities, but with concomitant environmental noise. This work is reviewed by Giallorenzi, et. al. (1982).
The use of optical fiber as a transmission link in underwater sensing is relatively new, but has had several successful applications. Gregg, et. al. in 1982 made use of the high data rate capability of optical fiber to service six electrical sensors in a free-fall microstructure profiler; Lund beginning in 1983 uses optical fiber for in-situ algae mapping by stimulating and detecting fluorescent emissions; and the Naval Ocean systems Center established the feasibility of using optical fibers for expendable communications links in 1982. A caution to this, however, was added by S. Hanish in 1981 at the Naval Post Graduate School in Monterey; he found that thermal and mechanical stresses produced by the ocean environment created a moderate to severe effect on phase sensing. Remote oceanic interferometric sensing techniques are not currently practical.
Each of the foregoing prior art devices are useful in particular applications. However, it would be an advancement in the art to provide a combination of temperature, pressure, and index of refraction sensors that were accurate and free from drift, that could be used to make measurements in remote and inaccessible locations such as the oceans, and that could even be expendable. Such a unique combination of sensors, detector, and transmission link is disclosed and claimed herein.